1. Field of the Invention
The present invention relates to an exchange coupling thin film used for reading magnetic information from a magnetic recording medium by employing a magnetoresistive effect based on magnetic exchange coupling, and a magnetoresistive element and a magnetic head each comprising the same.
2. Description of the Related Art
Conventional magnetoresistive reading heads (MR heads) of prior art include AMR (Anisotropic Magnetoresistance) heads, and GMR (Giant Magnetoresistance) heads employing the spin-dependent scattering of conduction electrons. As an example of the GMR heads, U.S. Pat. No. 5,159,513 discloses a spin-valve head exhibiting a high magnetoresistive effect in low external magnetic fields (abbreviated to a "spin-valve head" hereinafter).
FIGS. 1 and 2 are schematic drawings of the structure of an AMR head element. In order to optimally operate an AMR head, two bias magnetic fields are required for an AMR ferromagnetic film 3 (AMR material layer) exhibiting the AMR effect. One of the bias magnetic fields is used for causing the resistance of an AMR material to change in linear response to a magnetic flux, and is perpendicular (in the Z direction shown in the drawings) to the plane of a magnetic medium and parallel to the film surface of the AMR ferromagnetic film 3. This bias magnetic field is generally referred to as a "lateral bias", and can be obtained by arranging a soft magnetic film 1 formed near the AMR ferromagnetic film 3 through an electric insulating layer 2 and passing a detection current through the AMR head from a conductive layer 5.
The other bias magnetic field is generally referred to as "longitudinal bias", and applied in parallel (in the X direction shown in the drawings) with the magnetic medium and the film surface of the AMR ferromagnetic film 3. The longitudinal bias magnetic field is applied for the purpose of suppressing the Barkhausen noise generated by the many magnetic domains formed in the AMR ferromagnetic film 3, i.e., for the purpose of causing a smooth change in resistance with the magnetic flux from the magnetic medium without noise.
In order to suppress the Barkhausen noise, it is necessary to form a single magnetic domain in the AMR ferromagnetic film 3. This is achieved by the following two methods for applying the longitudinal bias.
One method employs a leakage magnetic flux from the magnet layers 6 arranged on both sides of the AMR ferromagnetic film 3 (on both widthwise sides of the ferromagnetic film 3 formed to have a width corresponding to a track width shown in FIG. 2), as shown in FIG. 2. The other method employs the exchange anisotropic magnetic field generated in the contact interface between the ferromagnetic film 3 and the antiferromagnetic films 4 spaced on the AMR ferromagnetic film 3 in correspondence with the track width, as shown in FIG. 1.
On the other hand, in order to optimally operate the spin-valve head, in a sandwich structure comprising a free ferromagnetic film 7, a non-magnetic intermediate layer 8 and a pinned ferromagnetic film 9, as shown in FIG. 4, it is necessary that a bias in the track direction (in the X direction shown in the drawing) is applied to the free ferromagnetic film 7 so that the direction of magnetization is in the track direction with the single magnetic domain formed, and a bias in the Z direction shown in the drawings, i.e., in the direction perpendicular to the magnetization direction of the free ferromagnetic film 7, is applied to the pinned ferromagnetic film 9 so that the magnetization direction is in the Z direction shown in the drawing with the single magnetic domain formed. The magnetization direction of the pinned ferromagnetic film 9 must not be changed by a magnetic flux (in the Z direction shown in the drawings) from a magnetic medium, and the linear response of the magnetoresistive effect can be obtained by changing the magnetization direction of the free ferromagnetic film 7 within the range of 90.+-..theta. degrees with respect to the magnetization direction of the pinned ferromagnetic film 9.
In order to fix the magnetization direction of the pinned ferromagnetic film 9 in the Z direction shown in the drawings, a relatively large bias magnetic field is required, and the bias magnetic field is preferably as large as possible. A bias magnetic field of at least 100 Oe is required for overwhelming a diamagnetic field in the Z direction shown in the drawings and preventing the magnetization direction from fluctuating due to the magnetic flux from the magnetic medium.
A general method for obtaining this bias magnetic field is to employ the exchange anisotropic magnetic field produced by bringing an antiferromagnetic film 10 in contact with the pinned ferromagnetic film 9, as shown in FIG. 3 or 4.
The bias applied to the free ferromagnetic film 7 is adapted for ensuring the linear response and suppressing the Barkhausen noise generated by formation of many magnetic domains. As the method of applying this bias to the free ferromagnetic film 7, the same methods as those for applying the longitudinal bias to the AMR head, i.e., the method employing the leakage magnetic flux from the magnet layers 11 arranged on both sides of the free ferromagnetic film 7, as shown in FIG. 3, and the method employing the exchange anisotropic magnetic field generated in the contact interface with the antiferromagnetic films 13, as shown in FIG. 4, can generally be used.
As described above, the exchange anisotropic magnetic field generated in the contact interface with the antiferromagnetic films is employed for applying the longitudinal bias to the AMR head or applying the bias to the pinned ferromagnetic film and applying the bias to the free ferromagnetic film of the spin-valve head. As a result, a magnetoresistive head having good linear response and suppressed Barkhausen noise can be realized.
The exchange anisotropic magnetic field is the phenomenon caused by the exchange interaction between the magnetic moments of a ferromagnetic film and an antiferromagnetic film in the contact interface therebetween. As the antiferromagnetic film which produces an exchange anisotropic magnetic field for the ferromagnetic film, e.g., an NiFe film, an FeMn film is known well. However, the FeMn film has very poor corrosion resistance and thus has the problem that corrosion occurs and proceeds during the magnetic head manufacturing process and operation of a magnetic head, thereby deteriorating the exchange anisotropic magnetic field and damaging the magnetic medium. In addition, during the operation of the magnetic head, the temperature in the vicinity of the FeMn film is known to increase to about 120.degree. C. due to the heat generated by the detection current. However, the exchange anisotropic magnetic field generated by the FeMn film is sensitive to temperature changes, and substantially linearly decreases with temperature rises to about 150.degree. C. (blocking temperature: Tb) at which it disappears. Thus there is also the problem of making it impossible to obtain a stable exchange anisotropic magnetic field.
As an invention relating to improvements in the corrosion resistance and blocking temperature of an FeMn film, for example, U.S. Pat. No. 5,315,468 discloses an NiMn alloy or NiMnCr alloy which has a face-centered tetragonal structure. However, the corrosion resistance of the NiMn film is higher than that of the FeMn film, but is insufficient for practical use. Although the NiMnCr film comprises an alloy containing Cr which is added for improving the corrosion resistance of the NiMn film, and the corrosion resistance is improved by adding Cr, the level of the exchange anisotropic magnetic field is decreased.